Need help locating newly adopted kitty (definitely still inside house)

[Digit]

Last Monday, I adopted a two year old female cat from the local animal shelter. As some of you know, I had to have my last kitty put down at the ripe old age of 20 when she was in liver and kidney failure.
This new kitty adores me and is by my side constantly.

I thought that maybe she needed a playmate so I answered an ad for a 1 yr old kitty that was not working out for the family. I brought her home yesterday afternoon and set her up in the half bath with a separate litter box, food and water. She escaped from there and is somewhere in my house. She managed to get behind the water heater in a rather inaccessible area, but I can't see her there any longer. There is duct work right by that and I don't know if she could get into that and under my house or what.

I have run the can-opener in the rooms she could be in, but no response. I even looked under the house and heard nothing. Under the house is all brick and if she did get into the crawl space, there would be no escape unless I opened the door.

All bedrooms were blocked off already except for my own, and I have already checked mine thoroughly (twice) and then shut the door.

I have not been outside today or yesterday after coming home, so I am positive she has not gotten out.

There is a family room, living room, dining area (no place to hide), kitchen, pantry (I have checked) and half bath (washer and dryer are here and folding doors were shut), plus that small closet with the water heater.

I am getting very concerned she might be stuck somewhere.
Does anyone get a sense of where she might be? The other kitty is helping me look, but she can't find her either.